1. Field of the Invention
The present invention relates to a torque limiting device for motors in electrically driven vehicles and an electrically driven vehicle incorporating the torque limiting device. More particularly, the present invention relates to a torque limiting device which is suitable for limiting the torque so that the motor as a driving power source may not rotate at a speed exceeding a predetermined value in electrically driven vehicles such as an electrically driven bicycle, an electrically driven wheelchair, and an electric automobile, and which is especially suitable for use in an electric bicycle often referred to as an "electrically assisted bicycle" in which the human driving power is assisted by a motor driving power.
2. Description of the Related Arts
Recently, electric motorcars are attracting public attention. Also, popularity is enjoyed by electric bicycles having both a human power driving section utilizing a human power and an electric power driving section utilizing a motor so that the motor is driven in accordance with the magnitude of human driving power to assist the human driving power with the motor driving power.
Such an electric bicycle is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. Hei 6(1994)-107266. Namely, it is generally known in the art that a vehicle speed signal detected by a vehicle speed detection means such as a speed sensor provided in a wheel and a pedalling signal which is a human driving power detected by a pedalling power detection means are inputted into a controller whereby the assist ratio, namely the ratio of the motor driving power to the human driving power, are varied in accordance with the vehicle speed on the basis of predetermined table data.
In such an electrically assisted bicycle which outputs a motor output in accordance with the human driving power, a control as shown in the conceptual view of running of FIG. 4 is performed in order to protect the user from the risk of gaining excessive speed. Namely, the assist ratio which is a ratio of the motor driving power relative to the human driving power is maintained within a safety range by setting the assist ratio to be 1 when the bicycle speed is 15 km/h or less, allowing the ratio to linearly decrease from 1 to 0 when the bicycle speed is more than 15 km/h and less than 24 km/h, and setting the ratio to be 0 when the bicycle speed is 24 km/h or more to stop the driving by the motor driving power. The above relation is illustrated by a graph in FIG. 5.
For that purpose, the electrically assisted bicycle includes a torque sensor for detecting the torque of the human driving power, a speed sensor for detecting the running speed, and a microcomputer for performing calculations to determine the motor driving power, whereby the torque of the human driving power obtained by the torque sensor and the running speed obtained by the speed sensor are inputted to change the assist ratio on the basis of the table data stored in the microcomputer.
For example, supposing that a torque of 100 kg.multidot.cm based on the human driving power is applied, the torque based on the motor driving power is set to be 100 kg.multidot.cm when the running speed is 10 km/h, whereas an output of 44 kg.multidot.cm based on the motor driving power is calculated in the microcomputer and outputted when the running speed is 20 km/h because the assist ratio must be reduced to about 0.44.
However, if the speed sensor has errors or if the speed sensor is out of order, the real output may possibly exceed a predetermined value to give an excessive assist ratio or may possibly be too much smaller than the predetermined range to provide a sufficient assist because the speed determined by the speed sensor is different from the real running speed although the microcomputer performs calculations correctly.
Also, since the speed sensor must be provided in a driving wheel or the like which is a body of revolution, a wiring is needed, making the structure complex, or the wiring itself poses an obstacle.